The present invention is a method for measuring the fiber angle in a solid fibrous material relative to any of three mutually orthogonal reference axes. The method is particularly well adapted for measuring fiber angle in wood and wood products for use in conjunction with an automatic grading or stress rating system.
Wood is a highly anisotropic material composed of bundles of several types of lignocellulosic fibers. The great majority of these fibers are formed generally parallel to the pith or longitudinal axis of the tree. However, growth anomalies can greatly distort this parallelism in localized areas. One such anomaly is twist. This is a situation in which the growing tree lays down fibers in a helical fashion around the pith. A better known anomaly is caused by knots. A knot is caused when a growing tree forms new wood around an existing live or dead branch. A further example is caused by wounds which injure the cambium tissue under the bark. These cause the growing tree to lay down a form of scar tissue in which fiber direction can vary wildly.
Fiber direction, more commonly termed grain direction, has a significant effect upon the appearance and strength of the wood. Many "figured" hardwoods owe their appearance to small scale variations in fiber direction. One example would be the well known "fiddleback" pattern caused by the fibers being laid in a sinuous, rather than linear, pattern. Fiber angle variations are particularly important in wood destined for structural or construction uses. In many uses where stresses tend to induce bending, the allowable load is directly dependent on the uniformity and linearity of the fiber orientation in the member. Ideally, in construction wood, all of the fibers would be parallel to the longest axis of the member. This ideal is at best only approximated in the highest grades of lumber, which are essentially free from defects such as knots. The probable presence to some degree of less readily seen defects, such as those that might be caused by spiral grain, cause allowable stress ratings to be assigned very conservatively within any grade of lumber.
Lumber grading is normally carried out visually, using a well defined set of rules agreed upon throughout the wood products industry. In recent years, some lumber has been machine stress rated in addition to being visually graded. This is normally done automatically in a device that bends the individual pieces of lumber as a plank; i.e., with the load applied to the broadest face. Either a constant load is applied and the deflection measured, or a variable load is applied to achieve a given deflection. In either case, the property calculated is the modulus of elasticity. This value correlates with the flexural strength, also called the modulus of rupture, when the member is used as a joist; i.e., with the load applied to the narrower cross sectional dimension.
Machine stress rating used in conjunction with visual grading has been of considerable value to the user who has applications that have a critical dependence upon the strength of the wood. One such application might be the manufacture of glued laminated beams which frequently serve as major structural members in large buildings. Even so, the forces applied during machine stress rating are far short of those that would generally cause failure and they frequently fail to discriminate against potential weak areas caused by certain types of grain anomalies. One of these anomalies is a major fiber deviation from parallelism with the longitudinal axis. This can occur even in the absence of a knot or similar defect. In particular, the defect known as "cross grain" or "diving grain" is often very difficult or impossible for a grader to see. Diving grain, broadly stated, is the aforementioned phenomenon in which the fiber direction is not parallel to the longitudinal axis of a member, but is either angled upward or downward in reference to the plane defining at least one face of the member, when the member is of rectangular cross section. Diving grain may also be present in wood products having other geometric cross sections, such as circular.
Diving grain is always associated with knots but it can also be present without any other visible defects being present. It must be remembered that a typical sawlog is a truncated cone due to the natural taper present in most logs. Sawmill limitations which prevent sawing lumber parallel to the outer surface will produce diving grain to some degree. The ability to detect and measure it by a scanning system opens broader opportunities for more accurate stress rating as well as more comprehensive types of machine grading.
The concept of optical grading of wood is in itself not new. As one example, U.S. Pat. No. 3,976,384 to Matthews et al., describes an optical system concerned with the detection of defects such as knots, blue stain, and certain types of rot. These inventors "inject" light into the surface of wood at one point and measure the emerging light at an adjacent point. They have noted that in clear (defect free) wood, light traveling across the grain is attenuated by a factor almost 50 times greater than light traveling along the grain. The method senses surface fiber direction changes on the plane of the face being measured but is unable to satisfactorily detect diving grain or accurately measure surface grain direction. In addition, it had been found to be unsuitable for use on the wood of deciduous, or so-called "hardwood", species.
In French Pat. No. 2,499,717, a wood surface is illuminated with a polarized beam of light having the plane of polarization either parallel to or perpendicular to the longitudinal axis of the piece of wood. The light reflected from a piece of sound wood is only partially depolarized while light reflected from a knot is almost totally depolarized.
Davis et al., in U.S. Defensive Publication No. T932,008, disclose an optical system for measuring surface fiber direction in a moving web structure. These authors note that when collimated light is projected on a fibrous web, the light will be reflected with greatest intensity perpendicular to the fiber axes. To take advantage of this effect they illuminate a spot on the moving surface and measure the reflected light in the machine direction, in the cross machine direction, and at 45.degree. to the machine direction. A spot of collimated light is projected vertically onto the surface with the sensors being located at an angle of 45.degree. from the vertical. The degree of illumination is controlled to maintain a constant reflectance. Reflectance values measured along and across the machine direction can be used to determine the average orientation of the fiber.
The three systems just described are useful for detecting, and in one case measuring, deviations in surface fiber from a longitudinal orientation. Stated differently, they are limited to describing the fiber condition on a planar surface but unable to describe fiber orientation with respect to the three axes of a solid material. As an example, in wood they could detect and measure fiber surface angle only but would be unable to supply any information as to whether or not diving grain was present.
Certain other investigators have devised apparatus suitable for discovery of subsurface anomalies in translucent materials. Mullane, Jr., in U.S. Pat. No. 4,184,175, describes equipment useful for detection of subsurface flaws in teeth. Vukelich et al., U.S. Pat. No. 3,574,470, describe void detecting apparatus for a material such as polyurethane foam. Light is beamed into the material and the reflectance is measured. Areas having internal voids will reflect less light. Young, in U.S. Pat. No. 4,286,880, and Idelsohn, in U.S. Pat. No. 4,149,089, describe flaw detection systems for wood. Each of these systems also requires the presence of a human inspector who works in cooperation with the scanning system.
Other optical scanners are shown in patents to Hamada et al., U.S. Pat. No. 4,403,294 and Watson et al., U.S. Pat. No. 3,694,658.
Lucas et al., in U.S. Pat. No. 4,092,068, describe a scanner for determination of paper surface roughness and the detection of dirt. The device is especially sensitive to dirt particles because their reflectivity is very different from that of the paper. Lucas illuminates a circle from about 0.1 to 0.2 mm in diameter and uses two angularly spaced detectors to measure reflected light. Greer et al., in U.S. Pat. No. 3,591,291, and Tittmann et al., U.S. Pat. No. 4,274,288, teach methods for measuring the depth and frequency of surface flaws in objects.
Dahlstrom et al., in U.S. Pat. No. 3,983,403, teach a scanner system useful for detecting and measuring wane in lumber. In an invention that is somewhat related to the disclosure of Davis et al., Van Veld, in U.S. Pat. No. 3,471,702, notes that the ratio between diffuse and specularly reflected light correlates with the bulk of a strand of yarn. One detector is focused along the line of specular reflection and a second detector is located 171/2.degree. off this axis.
While all of the above scanning systems are undoubtedly useful for the purposes described, none of them have the ability to "see into" a fibrous three dimensional material so as to measure fiber angle with respect to all three axes. As will henceforth be described, the present invention has overcome this major limitation.